A photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term “photoelastic effect” means that an optical element that is mechanically stressed and strained (deformed) exhibits birefringence that is proportional to the amount of deformation induced into the element. Birefringence means that the refractive index of the optical element is different for different components of a beam of polarized light.
A PEM includes an optical element, such as fused silica, that has attached to it a transducer for vibrating the optical element. The transducer vibrates at a fixed frequency within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended along the axis of the optical element as a result of the vibration. The combination of the optical element and the attached transducer may be referred to as an optical assembly. The axis about which the optical element vibrates is referred to as the optical axis of the PEM.
The optical assembly is mounted within a housing or enclosure that normally includes an aperture through which the light under study is directed through the optical element in a direction generally perpendicular to the optical axis of the PEM. The enclosure supports the optical assembly in a manner that permits the optical element to be driven (vibrated) within it to achieve the above-noted photoelastic effect.
PEMs are commonly used in measuring polarization properties of either a light beam or a sample. Many instruments use two or more PEMs to provide measurements of certain polarization properties. When two PEMs are used in a single instrument, they are typically arranged so that their optical axes are oriented to be precisely 45 degrees apart (as considered in a direction perpendicular to those two optical axes).
Examples of typical, two-PEM instruments include complete Stokes polarimeters, Tokomak polarimeters, and a number of other polarimeters and ellipsometers. When four PEMs are used in one instrument, the PEMs are typically grouped in separate pairs.
The speed and precision with which a pair of PEMs can be oriented so that their optical axes are fixed at a particular, selected angle depends greatly on the precision with which the housing or enclosure to which the PEMs are mounted can be positioned and secured to place the PEMs in that proper orientation.